Selection when traits have different genetic and phenotypic variances in different environments.

Falconer's concept that performance in environment 2 is a different trait from performance in environment 1 allows calculation of expected response in environment 2 if selection is from environment 1. Response to selection in environment 1 and correlated response in environment 2 depend on heritability and phenotypic variance in environment 1, genetic covariance between performance of identical genotypes in the two environments, and selection intensity. If selection is from performance in environment 2, direct response in environment 2 and correlated response in environment 1 also can be calculated. If selection is from animals in both environments and if selected genotypes are expressed randomly in both environments, relative responses in environments 1 and 2 are weighted averages of direct and correlated responses with the weights being p1 and p2, the fractions of animals selected from environments 1 and 2. Fraction selected from one environment determines the selection intensity factor for the direct and correlated responses in that and the other environment. Other terms determining relative responses are independent of fractions selected. A simple approach to finding the optimum fractions, p1 and p2, is to calculate weighted average responses for environments one and two for all combinations of p1 + p2 = p, a fixed fraction.